For the operation of movable machine parts, one of the most important considerations is the simplification and reduction of cost of the drive and, particularly for machine elements of spinning and twisting machinery, it is advantageous to utilize drives which are not controllable with respect to speed because the increase in complexity is concomitant with a decrease in reliability in many instances.
It is, therefore, an advantage in spinning and twisting machines for the control of movable machine elements to utilize electric motors as the prime movers of such drives and to effect control in the simplest and most economical manner. Drives are used for a variety of machine elements in spinning and twisting machines. For example, in the apparatus described in German Patent Document DE 40 05 418 C1 the moving machine element is provided for doffing the bobbins from and applying the bobbin-winding cores and sleeves onto-the spindles or work stations of the spinning or twisting machine. The apparatus utilizes a movable machine element which must be displaced into predetermined positions and an absolute value sensor is provided to detect the position of the machine element and to signal the actual position to the control unit. The control unit then compares the actual position to the setpoint of the position approached by the machine element and controls the drive to bring the machine element to that desired position.
When, however, nonregulatable drives, i.e. drives whose speeds are not controllable, are used, it is not unknown that the setpoint position to be approached by the machine element will be overrun when the drive is deactivated upon coincidence of the actual position with the respective setpoint position. This is especially the case when the movable machine element has considerable mass with a correspondingly large inertia since there is a finite time after deactivation of the drive before the machine element is brought to standstill and, within this period, the inertia can cause significant travel of the machine element.
A solution to this problem is to inactivate the drive when the actual position of the machine element coincides with a point corresponding to the setpoint position reduced by a predetermined magnitude.
Even with such a control process, intolerable inaccuracies can occur with approach of the machine element to the setpoint position as a consequence, for example, of changes in the moving mass, of contamination of the bearings, of temperature-dependent variations in the viscosity of lubricants, etc.
The use of such drives is especially problematical in cases in which the movement of the machine parts must be effected with different but constant speeds since, upon a rapid movement of a machine element, the energy stored in the moving parts is significantly greater than is the case with more slowly moving machine elements so that the overrun differs also with respective speeds.
The problem arises not only with setpoints which are generated by calculation or measurement, but also where so-called "teach-in" processes are used to program the setpoint position. Such a process for programming the setpoints of an apparatus for doffing bobbins and applying the bobbin-winding cores or sleeves on a spinning machine or twisting machine is known, for example, from DE 40 05 418 C1. In this process, the setpoints are recorded in memory by a run through of the movement of the machine element under manual control and by recording the positions of the setpoints manually as well. The speed is substantially reduced during the teach-in run through. However, because of the inertia in the moving elements of the drive, inaccuracies upon approach to the setpoint position can arise when the machine elements are moved at higher operating speeds.
The other effects like contamination of bearings, mass changes and the like also generally cannot be taken into account during the teaching process.
Mention should also be made of German Patent Document DE 40 21 800 A1 which describes a process for controlling a positioning unit of an automated or other apparatus in which the approach to the setpoint position is effected through the use of a setpoint curve depending upon the distance between the art and target positions. This, of course, requires a speed-regulatable drive of greater complexity and cost than the drives with which the invention is concerned. German Patent Document DE 39 19 687 C2 describes a spinning machine with individual spindle drives whereby each individual drive is coupled with the auxiliary units of the spinning machine via a processor-controlled unit. This ensures an exact matching of the speed of the drive for the auxiliary unit especially the drafting velocity to the speed of the individual spindle drive. The result is an improvement in the yarn quality or the generation of yarns with different features.